Generally speaking, a bearing comprises an inner ring and an outer ring adapted to rotate around a rotation axis, one with respect to the other. In a plain bearing, the two rings are in sliding contact. In a rolling bearing, several rolling bodies are installed between the two rings. These rolling bodies can be balls, rollers or needles. Thus, a rolling bearing can be, for instance, a ball bearing, a roller bearing or needle bearing.
In the field of bearings, it is known to use a tachometer in order to determine the rotation speed of a member supported by a bearing. As explained in EP-A-1 933 155, one can use an encoder washer with magnetic poles fast in rotation with a rotating ring of a bearing, and one or several sensors distributed around the encoder washer or located in front of it. An air gap is provided between the encoder washer and the or each sensor and detection of the rotation parameter occurs through this air gap, thanks to the variation of a magnetic field generated by the encoder washer.
Such a detection device can be used in a magnetically perturbed environment such as in the neighbourhood of an electric motor where high magnetic fields are generated between the stator and the rotor. Under such circumstances, the accuracy of the measure performed by the sensor or sensors can be affected by a surrounding perturbing magnetic field, in particular because of electromagnetic interference or EMI.
U.S. Pat. No. B-6,741,073 discloses several complicated devices meant to build a magnetic bypass in order to avoid influencing a magnetic sensor. This does not protect the sensor from a surrounding perturbing magnetic field.
U.S. Pat. No. B-7,249,891 teaches the use of a conductive member having an electromagnetic shielding effect on one side only of a sensor holder, which does not efficiently protect a sensor from a surrounding perturbing magnetic field.